This invention relates generally to electrical machines, such as generators and motors and more particularly to a laminated pole for an electrical machine with well-defined magnetic poles, i.e., salient field poles, in which the laminated pole body is braced by pole end plates and a prestressing means, and to a method for making a laminated pole.
In conventional electrical machine with salient poles, the rotor poles are either solid or laminated depending upon the magnitude of the mechanical stress that the pole is to be subject to. Solid poles made from forged or cast steel have relatively small ohmic resistance and therefore suffer from relatively high losses. Such losses include eddy current losses produced by the higher field harmonics generated by the armature current, as well as the pulsation losses caused by the main field through the grooving. The solid poles act as the conductor rods of a squirrel cage rotor winding, and therefore, with asynchronous operating conditions, large currents are able to flow in the poles. Since the solid pole is able to conduct large currents and is able to absorb a large amount of power, the solid pole is especially suited for the asynchronous starting of large synchronous motor generators.
Laminated poles have a relatively higher ohmic resistance in the longitudinal direction than solid poles. The attendant losses are therefore correspondingly less. However, the damping effect of laminated poles is also correspondingly less. Therefore, a damping winding is provided with a laminated pole in order that the required operating stability be achieved.
A laminated pole is fabricated from punched sheets, each sheet having a thickness of about 2 millimeters. The sheets are layered one against another in a press over a mandrel. An axial pressure is exerted against the sheets, and after a high axial pressure is reached the mandrel is welded on both ends to solid pole end plates. Additional welding seams are then added on the outside contours of the sheets while the sheets are still under pressure. If the punching or layering operations contain steps to provide for the construction of carrying catches, ventilation slots, supporting dogs for the pole windings and so forth, no further work need be done after the additional welding seams are imparted to the sheets. See, for example, "Synchronmaschinen fur Wasserkraftanlagen" ["Synchronous Turbines for Hydraulic Power Stations"], CH-T 130 082 D, 1977, BBC Aktiengesellschaft, Brown, Boveri & Cie., Baden, Switzerland, at page 32.
The manufacturing processes for making pole end plates for slot-running synchronous machines from a solid steel is relatively expensive. This is especially so for hydro-generators having a large number of poles. In addition to high cost, the losses associated with solid poles and thus solid pole end plates have an adverse affect upon the operating behavior of the machine.
It is therefore an object of this invention to provide a laminated pole for an electrical machine with salient poles that is lower in loss than conventional poles, and is simple and economically feasible to produce.
According to a preferred embodiment of the present invention, the pole end plate is laminated. This reduces the manufacturing cost of the pole by about 40 to 50% when compared to the cost of using solid pole end plates. Additionally, lamination results in a pole end plate that is lighter in weight, since, due to the lamination, the filling factor is .ltoreq.98%. The reduced weight is accompanied by a reduction in the mechanical stress experienced by the pole anchoring mechanism. Still further, the pulsation losses and eddy current losses of the laminated pole end plate are less than those of a solid end plate.